1. Field of the Invention
The present invention relates to a structure of a vibration motor which utilizes a travelling surface wave.
2. Description of the Prior Art
A vibration wave motor transduces a vibration motion created by application of a periodic voltage to electrostrictive elements to a rotational motion or a linear motion. Because it does not require windings as opposed to a conventional electromagnetic motor, it is simpler and sma11er in structure and produces a high torque at a low rotating speed.
FIGS. 1 and 2 show a principle of drive in a prior art vibration wave motor. FIG. 1 illustrates the generation of the surface wave in the motor. Numeral 1 denotes a vibration member, and numerals 2a and 2b denote electrostrictive elements which are bonded or welded to the vibration member 1 (usually made of metal) and arranged on one side of the vibration member 1 with a spatial phase difference of .lambda./4 therebetween.
The vibration member 1 is used as one electrode for the electrostrictive elements 2a and 2b and an A.C. voltage V=V.sub.0 sin .omega.t is applied to the electrostrictive element 2a from an A.C. power supply 3a while an A.C. voltage V=V.sub.0 sin (.omega.t.+-..pi./2) having a phase difference of .lambda./4 is applied to the electrostrictive element 2b, where signs + and - are selected by a phase shifter 3b in accordance with a direction of movement of the movable member 5. Let us assume that the sign - is selected and the voltage V=V.sub.0 sin (.omega.t-.pi./2) is applied to the electrostrictive element 2b.
When only the electrostrictive element 2a is vibrated by the voltage V=V.sub.0 sin .omega.t, a vibration by a standing wave is generated as shown in FIG. 1 (a), and when only the electrostrictive elements 2b is vibrated by the voltage V=V.sub.0 sin (.omega.t-.pi./2), a vibration by a standing wave is generated as shown in FIG. 1 (b). When the two A.C. voltages having the phase difference therebetween are simultaneously applied to the electrostrictive elements 2a and 2b, the surface wave travels.
FIGS. 1(A), 1(B), 1(C) and 1(D) show the surface waves at times t=2n.pi./.omega., t=.pi./2.omega.+2n.pi./.omega., t=.pi./.omega.+2n.pi./.omega.and t=3.pi./2.omega.+2n.pi./.omega., respectively, and the wavefront travels in X-direction.
Such a travelling surface wave includes a longitudinal wave and a lateral wave. Looking at a mass point A of the vibration member 1 as shown in FIG. 2, a longitudinal amplitude u and a lateral amplitude w make a rotating elliptic motion.
A movable member 5 is press-contacted to the surface of the vibration member 1 and it contacts only an apex of the vibration member. (Actually, it makes contact in an area having a definite width.) Accordingly, the vibration member 5 is driven by the longitudinal amplitude component u of the elliptic motion of the mass points A, A', . . . at the apex and it moves in an arrow direction N.
When the phase of the voltage is shifted by 90.degree. by the 90.degree. phase shifter, the surface wave travels in -X-direction and the movable member 5 moves in the opposite direction to the direction N.
A velocity of the mass point A at the apex is V=2.pi.fu (where f is a vibration frequency) and a velocity of the movable member 5 depends thereon and also depends on the lateral amplitude w because of the frictional drive by the press-contact.
The velocity of the movable member 5 is proportional to the magnitude of the elliptic motion of the mass point A and the magnitude of the elliptic motion is proportional to the voltage applied to the electrostrictive elements. The magnitude of the elliptic motion is also proportional to the areas of the electrostrictive elements 2.
Accordingly, as the areas of the electrostrictive elements 2 increase, the rotation speed and the torque are increased. In actuality, as shown in FIG. 3, when the electrostrictive elements 2a and 2b have the same area and are arranged symmetrically, a vacant area of 3.lambda./4 is created because the electrostrictive elements 2a and 2b are arranged with the phase difference of .lambda./4 therebetween.
Accordingly, an area of one wavelength at minimum is available for drive.
When the areas of the electrostrictive elements 2a and 2b are not equal so that as much area as possible is utilized (either the electrostrictive element 2a or 2b is additionally arranged in the vacant area of 3.lambda./4 shown in FIG. 3 to increase the area available for drive), the area not available for drive is one half of that described above, that is, an area of one half wavelength. However, in this method, it is difficult to adjust the amplitude of the standing wave generated by the vibrations of the electrostrictive elements 2a and 2b and it is difficult to generate a stable travelling surface wave having a constant amplitude.
In actuality, since the travelling standing wave generated by the electrostrictive elements 2a and 2b is attenuated by the internal friction of the vibration member 1, the amplitude at a distant point from the excitation point is smaller and the motion of the mass point on the surface of the vibration member 1 periodically changes at a period of .lambda./2 with the position on the surface when the travelling surface wave is generated. Since the number of contact points A, A', . . . in FIG. 2 changes and the positions of the contact points vertically change, the contact area changes and hence the efficiency is lowered or a noise is created.